The present invention relates to mats such as mats useful for mounting catalytic convertors and to methods for making mats.
It is known to catalyse oxidation or reduction of combustion products by passing the products into intimate contact with a catalyst. It is also known to remove unwanted entrained particulate matter by filtering a stream of fluid, e.g. gas.
Vehicle exhaust gases are treated to reduce the amount of noxious gases which are emitted to the atmosphere. Vehicles typically use a catalytic convertor (CC), such as close coupled or underbody petrol or diesel oxidation catalysts or selective catalytic reduction devices. Cars which use diesel as a fuel are typically fitted with a diesel particulate filter (DPF) to reduce the emission of small particles of soot and other materials produced during combustion.
Both CCs and DPFs are typically fabricated as ceramic monoliths through which the combustion products pass before they are emitted from the exhaust. The ceramic monoliths are fragile and relatively expensive. Accordingly, it is important to protect them from damage during use.
The monolith is located within a metal can mounted as part of a vehicle's exhaust system. As combustion products pass through the monolith they heat it, causing the monolith to expand. Of course, the can will also heat and expand. Clearly, as the two materials will heat and expand at different rates, there is a potential for relative movement between the can and the monolith. In the conditions found in a vehicle exhaust system there is also significant vibration which could also cause the monolith to become damaged if not securely held.
To ensure that monoliths are securely held they are typically wrapped in mounting mats. These mats may be formed using intumescent or non-intumescent materials. Similar materials may be used for other automotive thermal insulation applications, particularly those where gas flows may impinge thereon.
Non-intumescent materials may include fibres chosen from ceramic or glass fibres such as silica, alumino silicates, borosilicates, alumina, zirconia and the like. The fibres are usually held in a binder matrix to aid handleability, although additional and/or alternative consolidation techniques may be used, e.g. needling.
If present, the binder may be arranged to decompose and be burned off from the mat so as to allow the mounting mat to adopt a configuration to exert pressure on the monolith and the walls of the can to securely hold the monolith in place during use. It will be appreciated that the holding force will need to be maintained throughout thermal cycling regimes. Another factor which is important is the friction coefficient between can and mat and mat and monolith. Clearly, if the coefficient of friction is too low, then the mat and/or the monolith may slip relative to the can which may impair performance or lead to damage of the monolith.
Accordingly, it is desirable to have a mounting mat material which is thermally stable and which can compensate for differential expansion rates of the can and monolith whilst maintaining a minimum holding pressure on the monolith and having suitable frictional characteristics.
It is also important to consider that the mat should provide a significant resistance to fluid flow therethrough, while in situ between the monolith and the can. This is necessary to ensure that fluid flows preferentially (e.g. exclusively) through the monolith, thereby being exposed to the catalyst or filter.
Close coupled catalytic convertors are those catalytic convertors which are situated close to the engine where the exhaust gases for treatment are at a higher temperature than are those in a more conventional CC position close to, or towards, the rear of the vehicle. One advantage of close coupled CCs is that the greater heat from the exhaust fumes causes the catalyst to heat to its effective temperature more quickly, therefore making the CC more efficient, particularly when starting the engine from cold.
However, a problem associated with the provision of a close coupled CC is that the mounting mat is susceptible to erosion by the hot fluid flow, particularly at its upstream edge. Over time, this erosion has negative effects on the performance of the mat, for example reducing the holding pressure exerted by the mat on the monolith or allowing more fluid to pass through the mat rather than the monolith, i.e. bypassing the monolith.
It is therefore an object of the present invention to provide a mounting mat which is resistant to erosion, especially (although not exclusively) when deployed to hold a close coupled CC.
It is known to add a rigidising compound, such as colloidal particles, e.g. silica or alumina sols, to a mat before it is canned with a monolith in a close coupled CC system. This rigidising compound is used so as to mitigate the effect of the erosion of the mat, but because of its ‘rigid’ nature it also makes the mat much less flexible, thereby making it more difficult to wrap the mat around the monolith. Typically, then, the installer must apply the rigidising compound to the mat immediately prior to canning, such that the mat and monolith can be sealed in the exhaust system before the rigidising compound dries. This both is time-consuming and expensive and it requires installers to handle potentially hazardous chemicals. The latter issue increases safety and cost concerns for the installer. It also means that suppliers have to supply mats and the rigidising compound, thereby increasing costs to the supplier.
It is therefore an object of the invention to provide an erosion resistant mat which does not require a treatment to be performed by installers prior to installation and/or can be transported in a flat condition and/or is readily installable and/or providing a flexible mat and/or is not damaged or destroyed before or during the installation process.
Monoliths which are not of circular cross section, such as oval or racetrack monoliths, represent a further challenge to the production of erosion resistant mats. This is at least in part because the shape of the monolith dictates that the compression of the mat, measured as fibre gap bulk density (FGBD) will not be constant around the circumference of a mounted monolith. In turn, this leads to certain parts of the mat, such as those regions in which the mat is less compressed, being more susceptible to erosion.
It is therefore a further object of the invention to provide a mat which exhibits resistance to erosion when mounting a monolith which has non-uniform mounting characteristics.
The Pressure Performance (PP) of a mat relates to the pressure exerted by a mat on a monolith at a particular FGBD. It is a further object of the invention to provide a mat having an improved PP.
In this specification, the terms “erosion resistant” and “resistant to erosion” and similar phrases denote compounds, treatments or substances which have a greater resistance to erosion than an untreated fibre mat.